In general, push-button switches on various types of electronic devices such as mobile information terminals and AV devices are structured by covering a key sheet having push-button switches (key tops) on a circuit board on which contact switches are placed. To enhance operability in a dark place, backlighting is used in which light from internal light sources provided on the back of the key sheet are used to brightly illuminate the key sheet.
As an example of this type of illuminated push-button switches, Japanese Unexamined Patent Application Publication No. 2008-140766 describes a structure in which a light-guiding sheet (light guide) is provided between a circuit board on which contact switches (metal domes) are placed and a key sheet having key tops and a base sheet (elastic sheet). If a light-guiding sheet is provided like this, light can be passed through the interior of the light-guiding sheet and can be transmitted over the entire key sheet.
However, the structure described in Patent Document 1 is problematic in that, for example, each push-button switch is thickened by an amount equal to the thickness of the light-guiding sheet, the number of parts is increased, and the light-guiding sheet disposed between the contact switches and the key sheet reduces click feelings and thereby impairs the operation feeling. Accordingly, an improved structure is proposed in which a highly transparent resin film is used as the base sheet so that the base sheet also has an effect as the light-guiding sheet, enabling the operation feeling to be less likely to be impaired without the push-button switch thickness and the number of parts being increased.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-140766